Slow wave delay line structure

ABSTRACT

A slow wave guiding structure such as a helix delay line for electron interaction type devices of the traveling wave type is disclosed having diamond heat sink supports for substantial enhancement of power levels of operation. The supports are bonded to metallic members to dissipate thermal energy. The invention is applicable to numerous slow wave structures including ladder, ring-bar, meander, as well as interdigital types.

United States Harper et al.

[ 1 Dec. 11, 1973 Assignee:

Filed:

Appl. Nof: 283,460

SLOW WAVE DELAY LINE STRUCTURE Inventors: Robert Harper, Concord; David Zavadil, Newton Center, both of Mass.

Raytheon Company, Lexington,

Mass.

Aug. 24, 1972 US. Cl 315/35, 313/42, 317/234 A,

Int. Cl. H01j 25/34 Field of Search 315/3.5, 3.6;

References Cited UNITED STATES PATENTS Washburn, Jr 315/35 Collard Brous lversen Karol et al 315/35 11/1971 Marchese 315/35 OTHER PUBLICATIONS Sheet Gunn Oscillator & Thermal Considerations", Proc. IEEE, Vol. 50, 3/68, pps. 336-338 Improved Performance of Silicon Avalanche Oscillators Mounted on Diamond Heat Sink", Proc. IEEE, Vol.55, 9/67, pps. 1617 & 1618 Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Saxfield Chatmon, Jr. Att0rneyHar0ld A. Murphy et al.

[57] ABSTRACT A slow wave guiding structure such as a helix delay line for electron interaction type devices of the traveling wave type is disclosed having diamond heat sink supports for substantial enhancement of power levels of operation. The supports are bonded to metallic members to dissipate thermal energy. The invention is applicable to numerous slow wave structures including ladder, ring-bar, meander, as well as interdigital types.

12 Claims, 5 Drawing Figures I SLOW WAVE DELAY LINE STRUCTURE BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to slow wave support structures for traveling wave type electron interaction devices.

2. Description of the Prior Art Traveling wave devices typically incorporate a slow wave electromagnetic energy guiding circuit formed by a plurality of spaced periodic components. The helix exemplifies one such structure for amplifying electromagnetic energy at microwave frequencies by extracting kinetic energy from an adjacent high power electron beam. The high frequency energy wave travels along the slow wave structure at a velocity less than that of light to establish a synchronous interaction relationship with the electrons in the beam. Electric and magnetic fields of the traveling wave energy on the guiding structure induce perturbations in the electron beam to form electron packets or bunches and space charge waves as a result of the net exchange of energy. The electron beam becomes velocity and density modulated along the direction of trajectory to produce alternating high frequency energy of either the backward or forward wave type.

The energy slow wave guiding structure due to ohmic losses as well as electron bombardment is required to dissipate a considerable amount of thermal energy during the interaction process. Thermal energy dissipation is the primary impediment to attainment of higher average power levels in the electron interaction devices. This restriction is most significant at the higher microwave energy frequencies where the physical dimensions of the slow wave guiding structure and the beam are relatively small which results in an overall increase in the thermal impedances. Typically, prior art devices utilize slow wave structure supports of nonelectrically conductive materials such as, beryllia, boron nitride or ceramic having high thermal conductivity characteristics. Such materials are conventionally provided as elongated rods contacting the circular periodic components and extending parallel to the longitudinal axis of the device. Where the dimensions of the support structures are less than 0.030 to 0.040 inches granular materials such as beryllia become exceedingly brittle and subject to fracture due to thermal stresses.

A prior art electron interaction device having nonelectrically conductive supports to provide the required electrical properties as well as thermal dissipation is shown in FIGS. 4 and 5. The device 10 comprises a helix 12 having a plurality of turns extending along the longitudinal axis of a hermetrically sealed envelope 14 which may be of a metal or glass. The electromagnetic energy is coupled to the helix 12 by input conductor means such as a coaxial transmission line 16 and similar output means 18 are a coaxial transmission line 16 and similar output means 18 are disposed adjacent to a collector electrode 20.

A gun type electron beam source 22 includes an emissive cathode 24 having a slight concave curvature to assist in the focussing of the electron beam trajectory along the longitudinal axis path. The cathode is heated by a coil 26 and electrical leads extend through the envelope walls to provide for the connection of the components of the gun source to appropriate DC voltage supplies. An accelerator electrode 28 suitably biased by, for example, a positive voltage potential assists in the beam focussing. External magnetic field producing means 30 which may include any of the high coercive force permanent magnet materials such as samarium or platinum cobalt or an electromagnet surround envelope 14 and provide a longitudinal magnetic field parallel to the axis of the device.

The helix delay line structure 12 comprises a plurality of turns 32 of an electrically conductive wire and is supported by means of a plurality of elongated nonconductive members 34 disposed along the axial length of the device. Members 34 are typically of an insulating material having very high thermal conductivity characteristics such as, for example, alumina, beryllia or boron nitride. Such materials, however, have an inherent limiting factor in that dielectric loading of the slow wave guiding structure results if a substantial amount of the support material is utilized. The thermal conductivity characteristics of, particularly, beryllia is somewhat higher than the other ceramics with a value of 0.52 watts/Clem measured at 20C. Boron nitride, particularly of the isotropic variety is capable of handling higher thermal stresses to increase thermal energy dissipation. All the foregoing materials, however, leave much to be desired in the advancement of the art to achieve higher average power levels where thermal conductivity must desirably vary from about 10 to 30 watts/C/cm. It may be noted at this juncture that the thermal conductivity characteristic of copper is approximately 4 watts/C/cm.

SUMMARY OF THE INVENTION In accordance with the teachings of the invention thermal impedances can be substantially reduced and the thermal energy dissipation properties increased by as high as an order of magnitude through the utilization of spaced diamond heat sink support structures separately contacting each component of the overall slow wave structure. The dielectric constant of diamonds is approximately 5.58 which is lower than beryllia and, therefore, structures of high thermal conductivity and low dielectric loading may be realized. The thermal conductivity of several types of natural diamonds are considered in the range of from 9 watts/C/cm to about 26 watts/C/cm. In an exemplary embodiment of the invention the diamond heat sink support structures are individually bonded by metallizing and brazing techniques to radially extending metallic support rods which in turn engage the inner walls of the envelope of the device. Intermediate conduit means such as blow- I pipes may be disposed between the ends of the metallic supports and the envelope walls to provide the desired stressing during brazing and for efficient thermal heat dissipation engagement of the individual turns of the helix slow wavesstructure. After fabrication these conduits serve as a means for circulation of fluid coolant medium. In an exemp exemplary embodiment of the invention with an electron beam voltage of i5 kilovolts and operation range at Krmmd frequencies the individual diamond supports are provided with at least two parallel planar surfaces for bonding to the helix and rod. The diamond heat sink supports may also have two further parallel surfaces to define a substantially cubical configuration and reduce dielectric loading. Approximately a seven-fold increase in thermal energy dissipation characteristics over prior art support structures were recorded with the illustrative structure.

BRIEF DESCRIPTION OF THE DRAWINGS 7 Details of the invention will be readily understood after consideration of the following description of a preferred embodiment and reference to the accompanying drawings, wherein:

FIG. I is an isometric view partly in section of the illustrative embodiment of the invention with the external magnetic field producing means omitted;

FIG. 2 is a cross-sectional view of an alternative embodiment of the invention;

FIG. 3 is an isometric view of one of the diamond heat sink support structures having a cubical configuration;

FIG. 4 is a longitudinal cross-sectional view of a traveling wave interaction device having prior helix support structure; and

FIG. 5 is a cross-sectional view taken along the line 55 in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 3 an illustrative embodi ment of the invention will now be described. In FIG. 1 an isometric view of the pertinent structure germane to the invention has benn shown including a slow wave guiding structure comprising a helix delay line 40 having a plurality of turns 42 as well as the support structure. The remainder of the overall tube components have already been described with relation to the prior art shown in FIGS. 4 and 5 6 and include theelectron beam source, collector electrode as well as external magnetic field producing means and energy coupling means. Reference to such components has been omitted in this portion of the specification in order that attention may be focussed on the novel structure disclosed.

A plurality of diamond heat sink support members 44 having substantially planar surfaces 48 as shown in FIG. 3 provide the thermal interface for maintaining the positioning of the helix and permitting operation at higher average power levels. In this embodiment four such diamond support members 44 are illustrated contacting the turns 42 at spaced intervals approximately 90 apart. Certain commercial grades of gem quality natural diamonds have thermal conductivity properties varying from 10 to 30 watts/"Clem. Cost as well as availability will determine whether the selected diamond support members fall in the upper or lower portion of the overall available thermal conductivity range.

As shown in FIG. 3 the diamond support members 44 of the least expensive variety have an irregular outline with polished planar surfaces 48. Such surfaces provide for bonding to the helix turns and metallic rods 46 of a material such as copper. To assist in reducing dielectric loading two additional flat surfaces 49 may be provided by grinding or cutting to define a cubical configuration. In a traveling wave tube embodiment K ,,,,,,d frequency operation the helix delay line would illustratively have a diameter of approximately 0.080 inches and be wound of a 0.030 inches wide copper tape having a thickness of approximately 0.008 inches. The pitch of the helix would be approximately 0.0475 inches or 2l-turns per inch. For such an embodiment it was discovered that diamond support members having ground planar surfaces 48 and approximately 0.0475 inches apart and surfaces 49 also ground flat would be approximately 0.025 inches apart.

Surface 48 of each of the diamond support members 44 is joined by metallurgical techniques to the metallic rod members 46 of about 0.010 inch diameter. These members 46 in turn abut the outer ends of elongated hollow conduit members 50 such as blowpipes which in turn abut the metallic vacuum envelope 52. The aforementioned components are maintained in their respective positions within the tube envelope by means of circuit backwall member 54 also of a highly conductive metal such as copper provided with spaced holes 56 adapted to accommodate and radially support the rod members 46. Each of the blowpipes 50 may be provided with a substantial planar surface 58 to abut against the substantially planar outer ends of the rod members 46 and provide an efficient thermal interface.

A method of metallizing the diamond support members 44 follows. First, the diamonds are coated with sputtered titanium having a thickness of approximately 1,500 Angstroms followed by sputtered platinum of approximately 2,500 Angstroms thickness. A plating of gold 0.0002 inches thick completes the diamond bonding coatings. The copper rod members are also gold plated.

The abutting surfaces are desirably brazed in a vacuum atmosphere in the range of 10" to 10 Torr. A temperature range of 925 to 950C was found to provide the best diffusion bonding of the interface surfaces and successive brazes were fabricated utilizing gold. It is also possible to braze the components with a silvercopper eutectic alloy with 12 percent titanium by weight and heating the assembled components in an argon atmosphere.

Each pin of the prebrazed diamond heat sink and rod support assembly is silver plated and any metallizing or plating on the sides of the diamond support members is removed by sand blasting. The prebrazed assemblies are then positioned within the holes 56 until they bear against the mandril-supported helix delay line turns 42. The blowpipes 50 are then positioned to abut the rear ends of each of the rod members and outer envelope shell 52 is slid over the assembled components. The blowpipes are provided with a plated surface abutting the rods and the entire assembly is processed through a conventional brazing furnace. The braze is completed between the inner ends of the diamond support members 44 and helix delay lines turns 42 as well as between the rod members 46 and the blowpipe surfaces 58. The assembly is then ready for assembly of the remaining components including the cathode gun, collector electrode, and input and output coupling means, final exhaust and test. The blowpipe members 50 provide the necessary pressure to provide for bonding the component by the introduction of a heated fluid medium during brazing to substantially expand the walls and exert the desired pressure. After the device has been completely assembled the blowpipes 50 may be utilized as part of the overall fluid coolant circulation means to further dissipate thermal energy generated during the operation.

It is also possible to reduce dielectric loading in the alternative embodiment of the invention now to be described with reference being directed to FIG. 2. In this embodiment the helix delay line 60 is supported by three diamond heat sink support assemblies spaced apart approximately The support assemblies are disposed within aligned holes in circuit backwall member 62 abutting envelope 64 and extend radially. The

circuit backwall member is provided with channels 66 to accommodate the blowpipe members 68. Rod members 70 having brazed diamond support members 72 contact the turns of the helix 60 and the blowpipes 68. These components are brazed as described herein with reference to FIG. 1. Additionally, circuit backwall member 62 which is typically of a high thermal conductivity metal such as copper may also be provided with further channels 74 to lighten the weight of the overall embodiment by reducing the overall mass of the metallic materials and also to reduce the thermal impedances. The blowpipe members 68 are also provided with heated fluid medium to exert pressure during the brazing to assist in the bonding as well as the circulation of a coolant during operation of the traveling wave tube.

Other modifications and variations will be readily apparent to those skilled in the art and, therefore, the foregoing description of an illustrative embodiment is to be considered broadly and not in a limiting sense.

We claim 1. A traveling wave electron interaction device comprising:

a slow wave guiding structure for propagating electromagnetic wave energy;

means for generating and directing a beam of electrons along a path adjacent to said guiding structure to interact in energy exchanging relationship with said propagating wave energy;

combined means for supporting said guiding structure including diamond support members disposed at spaced intervals in contact therewith; and thermal energy dissipation means in contiguous relationship with said diamond support members; said dissipation means comprising metallic members having a high thermal energy conductivity characteristic.

2. The device according to claim 1 wherein said diamond members have at least two oppositely disposed parallel planar surfaces.

3. The device according to claim 1 wherein said diamond members have a substantially cubical configuration.

4. The device according to claim 1 wherein said ther- 6 mal energy dissipation means comprise metallic rods bonded at one end to said diamond support members.

5. The device according to claim 4 wherein the opposing ends of said metallic rods contact elongated metallic conduit means.

6.'The device according to claim 5 wherein said conduit means contain a circulating fluid medium coolant.

7. A traveling wave electron interaction device comprising:

an evacuated envelope having a longitudinal axis;

a slow wave guiding structure having periodically spaced components for propagating electromagnetic wave energy extending along said axis;

means for generating and directing a beam of electrons along a path adjacent to said guiding structure to interact in energy exchanging relationship with said propagating wave energy; and

combined means for supporting said guiding structure including diamond support members bonded to metallic members of a high thermal energy conductivity characteristic;

said diamond support members contacting said guiding structure components at spaced intervals; and

said metallic members are disposed within a metallic backwall member surrounding said guiding structure.

8. The device according to claim 7 wherein said guiding structure comprises a helically wound delay line having a plurality of spaced turns of an electrically conductive material.

9. The device according to claim 8 wherein said diamonds are thermally bonded to each of said spaced turns at spaced intervals.

10. The device according to claim 7 and elongated metallic conduit means contacting one end of said metallic members and said envelope.

11. The device according to claim 10 wherein said conduit means contain a heated fluid medium prior to completion of fabrication and a circulating fluid medium coolant after completion of fabrication.

12. The device according to claim 7 wherein said metallic members extend radially within said backwall member. 

1. A traveling wave electron interaction device comprising: a slow wave guiding structure for propagating electromagnetic wave energy; means for generating and directing a beam of electrons along a path adjacent to said guiding structure to interact in energy exchanging relationship with said propagating wave energy; combined means for supporting said guiding structure including diamond support members disposed at spaced intervals in contact therewith; and thermal energy dissipation means in contiguous relationship with said diamond support members; said dissipation means comprising metallic members having a high thermal energy conductivity characteristic.
 2. The device according to claim 1 wherein said diamond members have at least two oppositely disposed parallel planar surfaces.
 3. The device according to claim 1 wherein said diamond members have a substantially cubical configuration.
 4. The device according to claim 1 wherein said thermal energy dissipation means comprise metallic rods bonded at one end to said diamond support members.
 5. The device according to claim 4 wherein the opposing ends of said metallic rods contact elongated metallic conduit means.
 6. The device according to claim 5 wherein said conduit means contain a circulating fluid medium coolant.
 7. A traveling wave electron interaction device comprising: an evacuated envelope having a longitudinal axis; a slow wave guiding structure having periodically spaced components for propagating electromagnetic wave energy extending along said axis; means for generating and directing a beam of electrons along a path adjacent to said guiding structure to interact in energy exchanging relationship with said propagating wave energy; and combined means for supporting said guiding structure including diamond support members bonded to metallic members of a high thermal energy conductivity characteristic; said diamond support members contacting said guiding structure components at spaced intervals; and said metallic members are disposed within a metallic backwall member surrounding said guiding structure.
 8. The device according to claim 7 wherein said guiding structure comprises a helically wound delay line having a plurality of spaced turns of an electrically conductive material.
 9. The device according to claim 8 wherein said diamonds are thermally bonded to each of said spaced turns at spaced intervals.
 10. The device according to claim 7 and elongated metallic conduit means contacting one end of said metallic members and said envelope.
 11. The device according to claim 10 wherein said conduit means contain a heated fluid medium prior to completion of fabrication and a circulating fluid medium coolant after completion of fabrication.
 12. The device according to claim 7 wherein said metallic members extend radially within said backwall member. 